Why does my Rokinon 16mm f/2 lens cause underexposure?

Question

I'm into night sky photography and I recently picked up a Rokinon 16mm f2.0 lens for my Canon 70D. It appears that it's one of the best night sky lenses for APS-C cameras and I'm really excited to use it. It's a manual lens and I understand the need to adjust focus and aperture via the lens itself and not the camera.

What I'm confused about right now is that my camera doesn't seem to tell me the appropriate exposure at all prior to taking a picture. Basically, if I set my shutter speed to what my camera says is zero/in the middle/ perfectly exposed - the photo image when I view it is significantly underexposed. In order for me to expose properly where the histogram is in the middle - I actually need to take the picture at somewhere around +3 on my meter. This doesn't seem right to me, but then again I'm still very much a novice.

I bought the lens used, so most importantly I want to make sure that I didn't get hosed by a faulty lens (or maybe there is even something wrong with my camera? But I don't seem to have this problem with my kit lens).

This problem happens regardless of which metering mode I'm in. When I try aperture priority it doesn't seem to be an issue, but I don't plan on using aperture priority mode for anything with this lens.

Answer 1

If I'm not wrong it's a totally manual lens, so what the camera meter it's exactly what you'll shoot. And in this sense no, you shouldn't have this problem at all.

But given that, again, is totally manual and there is no electronic contact at all, there is nothing the lens can do to confuse the camera meter. So the only explanation that comes to my mind is that your metering is set in a way that prevent the camera to correctly measure the light, maybe like having the metering set to central spot and pointing the camera to a star...but you already excluded that.

Apart from that...it happened to me to have taken just a few shoots of the sky at night (not in sky photography at all), and they obviously have the histogram all to the left...I can't think a way to shoot at something so dark like the sky at night and have the istogram perfectly in the center. Are your shoots underexposed, or is you who are expecting something unrealistic?

Finally: no point in using Av mode, you have to shoot in M.

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Update after OP comment

You can try to do a small test:

1. Choose an easy subject
2. Mount the kit lens; Av mode, choose an aperture, shoot
3. Go to View mode and note down the shutter speed used. While you are there, just because "never say never", check the recorded aperture
4. Mount the manual lens, M mode, set the same aperture and shutter speed as before with the kit lens, shoot

The two images should be nearly the same, and the parameter stored in the images too.

If they are not...the lens must have some weird problem.

Answer 2

I deleted my other answer. Yes, there is something weird on my camera-manual lens combination.

There is no more manual lens than the one I am using. A 40 year old Nikon lens with an adapter on a Canon.

The camera is reading the light you just have on the sensor, no re-calculation or anything. If I close the diaphragm the poor camera just receives less light and makes the adjustment in Av mode.

Here is the strange part. If I use that lens on full aperture, the photo is underexposed compared with the same photo closing a bit the diaphragm.

My guess is that some vignetting is messing a bit on the exposure mode.

Try playing with it. Center exposed, Matrix, etc, and maximum aperture.

Answer 3

The night sky will tend to produce high luminance ratios [contrast] because empty space will approach black and celestial objects will approach white. In terms of the possible range of scenes a camera might be used to capture it is at an extreme edge.

A camera design may [and often does] trade off absolute fidelity in exchange for producing better results over most scenes. This means that exposure may be adjusted to preserve detail by preventing clipping at black and white. I suspect that your images are being adjusted to prevent clipping at the white, even though white clipping is likely to be less problematic in night sky images.

The solution is exposure compensation [it's common enough to be described in Manual of Photography, 10th edition, see table 12.2]. This article describes a complementary case of using exposure compenestion to overcome a camera's attempt to prevent clipping at black.

Incidentally, it is unlikely that the a camera system with interchangeable lenses will have a light meter in each lens. The usual camera design is to measure light as closely as possible to the image plane, and SLR's tend to use Through The Lens (TTL) metering for the built in system.